The Postal Numeric Encoding Technique (POSTNET) was developed by the U.S. Postal Service to provide an optimized bar code system for encoding zip code information on letter mail. The bar code is printed at the lower right-hand corner of the envelope. The basic elements of the code are tall and short bars, representing binary ones and zeros, respectively. Each five bars of the code represent a numeric digit. The ten digits are represented by the following combinations of tall and short bars:
______________________________________ 1 - 00011 6 - 01100 2 - 00101 7 - 10001 3 - 00110 8 - 10010 4 - 01001 9 - 10100 5 - 01010 0 - 11000 ______________________________________
The above combinations represent all possible combinations of two tall bars and three short bars.
A POSTNET bar field may have one of three formats containing 32, 52, and 69 bars, respectively. The 32 bit field is referred to as an "A field". The first and last bars are frame bars that are always tall bars. The remaining 30 bars represent a five digit zip code and a sixth digit that is a check character. The check character's value is such that the sum of the five zip code digits plus the check digit is a multiple of 10. For example, if the five digit zip code is 75028, then the check digit will have a value of 8, making the sum of all six digits a multiple of 10 (i.e. 7+5+0+2+8 +8=30).
The 52 bit field is referred to as a "C field". As with the A field, the first and last bars are frame bars. The remaining 50 bars comprise a nine digit zip code and a check character.
The third field type is actually two bar fields on a single envelope. If a mail piece already contains an A field, then a second field may be added to the letter to encode the remaining four digits of the zip code. The second field is added to the right of the A field and is referred to as a "B field". The B field comprises 37 bars, making a total of 69 bars for the A and B fields combined. As with the A and C fields, the first and last bars of the B field are frame bars, and are always tall bars. The first ten bars after the first frame bar of the B field represent the two digit zone of the nine digit zip code. The next four digits (20 bars) are the remaining four digits of the nine digit zip code. The last five bars before the frame bar are the check characters. Thus, the B field comprises six digits plus a check character. A B field always accompanies an A field and never exists by itself on an envelope.
The POSTNET bar fields are located approximately 1/4 inch above the bottom of the envelope. The A or C field begins about four inches from the right-hand edge of the mail piece. If a letter contains an A and a B field, the B field begins approximately 2.2 inches from the right-hand edge of the envelope. The bars in the field are spaced such that there are approximately 21 bars per inch. Each bar is approximately 0.02 inches wide. A tall bar is about 0.125 inches in height and a short bar is about 0.05 inches in height.
In the past, bar code readers have used black/white video data to determine the presence of bars based on the number of black cells in each scan of an optical scanner. Such readers, however, are sensitive to extraneous marks or smudges between the individual bars and in the bar fields. Furthermore, such readers have difficulty distinguishing bar codes from textual information. Thus, in the field of automated mail handling and sorting equipment there is a need for a bar code reader that can detect and read bar codes rapidly and accurately, even with the presence of smudges and written or typed information in the bar code field.